This invention relates to the reception of low frequency information amplitude modulating a high frequency carrier.
Amplitude modulation (AM) broadcast channel assignments set the frequency spacing between carrier frequencies of adjacent channels at 10 kHz, thus allowing each channel a 10 kHz bandwidth. The bandwidth for an AM double sideband signal is twice the bandwidth of the modulating signal. For example, if an AM carrier frequency is F.sub.c and the highest modulating frequency is F.sub.m, the bandwidth of the AM signal embraces lower and upper sidebands in the frequency range of F.sub.c -F.sub.m to F.sub.c +F.sub.m. To reduce interference of wide bandwidth transmissions, assigned AM channels in a local area are widely spaced. However, when transmission conditions are favorable, such as at night, distant AM signals can easily interfere with local signals. Interference of this type usually results in a 10 kHz whistle corresponding to the beat frequency with the carrier of the interfering station. To reduce audibility of this whistle, AM receivers typically include a filter cutting off audio frequencies above 3 kHz after the demodulation stage or in the IF amplifier. This filter prevents reproduction of higher spectral components, which are desired for high fidelity. Therefore, the receiver reduces the fidelity advantages of a wide bandwidth transmission.
The modulated signal can be recovered from either sideband. Single sideband (SSB) receivers allow selection of either sideband to reduce noise. Other AM receivers demodulate the sidebands separately and add the resulting demodulated signals. Still other AM receivers have 10 kHz bandpass filters on each sideband to detect the carrier of interfering stations, and provide variable band-reject filters for reducing the high frequency noise in sidebands adjacent to an interfering channel.
Stereophonic AM systems transmit signals with different spectral distributions in the two sidebands. Kahn (U.S. Pat. Nos. 3,218,393, 4,018,994, 4,641,341) and Ecklund (U.S. Pat. No. 4,489,431) disclose different AM stereo systems. The former system transmits left and right stereo information separately in the two sidebands. The latter system (the CQUAM stereo system) amplitude and phase modulates the carrier with the sum, and difference, respectively, of the stereo signals. Both of these stereo systems also modulate the carrier with a low frequency pilot tone, which indicates the presence of a stereo transmission. Kahn and Ecklund disclose using this pilot tone to reduce the effect of interference from neighboring channels through variable band-reject filters and channel mixing.